The present invention relates generally to a digital communication system for transmitting a digital signal over a metallic cable having a transmission attenuation characteristic .sqroot.f.
With the introduction of advancing technologies in data terminals, they are now capable of operating at a speed on the order of megabits per second. Various networks have been developed to allow efficient transmission of data between high performance data terminals. Most of these networks employ optical fibers as transmission media. Although satisfactory for transmitting such high speed data from data terminals to network access points, optical transmission media require the use of optical transceiver which significantly increase the cost of the data terminals. One simplest method to overcome this problem is to employ twisted wire pairs. However, signals transmitted on wire pairs attenuate significantly as the frequency of the signal increases as is known by the formula .sqroot.f.multidot.l, where f is the frequency of the signal being transmitted and l, the length of the transmission line. Multilevel signalling and partial response signalling are known as efficient methods for transmitting high speed data over a twisted pair of wires. However, multilevel signalling requires an adaptive equalizer at the receive end of the system to automatically suppress intersymbol interference (at sample points) which noticeably increases in applications which transmit signals having four levels or greater. Since the adaptive equalizer needs to perform discrete control on sample values if high precision operation is required, the harware necessary to implement such requirements would significantly increase in volume. Partial response signalling solves this problem. For a given number of signalling levels, a comparison between multilevel signalling and partial response signalling indicates that the latter is more advantageous for use with transmission lines having the .sqroot.f attenuation characteristic. However, the partial response technique overfilters the encoded signal and so it limits the spectrum of transmission narrower than the Nyquist bandwidth (f.sub.0 /2, where f.sub.0 is the symbol clock frequency). This results in a data bit stream having a small amount of clock components and makes it difficult for a nonlinear clock recovery circuit to generate the necessary timing signal.
From the clock recovery view point, bipolar coding technique is suitable. Since a signal is said to have ample clock components if it exhibits a high energy spectral density in the neighborhood of frequency f.sub.0 /2, the bipolar coded signal is the case in point. However, the bipolar signal has a greater range of mainlobe energy density than in the case of partial response signalling and so it requires a wideband equalizer, which results in a low signal to noise ratio. In addition, because of the three-level signalling, the bipolar coding adds complexity to the problem of signal to noise ratio.
FIG. 1 is a block diagram of a prior art digital communication system employing a (1, 1) partial response signalling scheme (which is known as a class-1 partial response signalling). An input binary digital data stream a.sub.n with symbol clock intervals T is passed through a precoder 45 formed by a delay line of length T and a modulo-2 adder and encoded with an intermediate data stream b.sub.n (where n is a sequence number identifying each symbol). The intermediate data stream b.sub.n is converted by a (1, 1) conversion circuit 46 into a (1, 1) multilevel data stream c.sub.n which is transmitted through a transmission line 47 to digitizer 48 at the receive end of the system. As shown in the drawing, this (1, 1) conversion circuit is made up of a delay line of length T and an adder. The (1, 1) multilevel data stream c.sub.n is converted by the digitizer 48 into a digital signal and fed to a transmission line equalizing filter 49 to compensate for the transmission loss. The output of the equalization filter 49 is applied to a decoder 50 where the input signal is converted to an output digital data stream d.sub.n which is a replica of the original data stream.
The following relations hold between the data streams a.sub.n, b.sub.n, c.sub.n and d.sub.n :
b.sub.n =b.sub.n-1 .sym.a.sub.n
c.sub.n =b.sub.n +b.sub.n-1
d.sub.n =[c.sub.n ].sub.mod2, (where d.sub.n =0 when c.sub.n is even, d.sub.n =1 when c.sub.n is odd), where .sym. represents modulo-2 summation.
If {a.sub.n }={101100101], then
{b.sub.n }={110111001]
{c.sub.n }={121122101], and
{d.sub.n }={101100101].
As shown in FIG. 2a, the spectral component of the (1, 1) partial response signalling code at one-half the clock frequency is significantly small, making it difficult for the receive end of the system to recover clock timing signals. On the other hand, the bipolar encoded signal has a spectral peak at one-half the clock frequency as shown in FIG. 2b, indicating that the bipolar signal is rich with clock timing information.
From the view point of signal to noise ratio in a .sqroot.f transmission line, the (1, 1) partial response signalling is advantageous over the bipolar signalling since the former needs only to detect the mainlobe of the spectrum at the receive end of the system, while the latter needs to detect a wideband mainlobe of the spectrum with a resultant decrease in signal to noise ratio. Therefore, the use of the partial response signalling to improve the signal to noise ratio results in a poor timing recovery performance, while the use of the bipolar signalling technique results in a low signal to noise ratio.